Wafer bonding by molecular adhesion is a technique that is well known per se. It should be recalled that the principle of wafer bonding by molecular adhesion is based on bringing two surfaces into direct contact, i.e., without using a specific bonding material (adhesive, wax, solder, etc). Such an operation requires the surfaces that are to be bonded to be sufficiently smooth, and free of particles or of contamination, and for them to be sufficiently close together to allow contact to be initiated, typically at a distance of less than a few nanometers. The attractive forces between the two surfaces are then large enough to cause bonding by molecular adhesion or “direct bonding” (bonding induced by the various electronic interaction attractive forces (Van der Waals forces) between atoms or molecules of the two surfaces to be bonded together).
FIGS. 1A to 1D show an example of the production of a multilayer structure comprising wafer bonding by molecular adhesion of a first wafer 102 onto a second wafer 106 constituting a support wafer.
The first wafer 102 includes a series of microcomponents 104 on the bonding face 102a (FIG. 1A). The microcomponents 104 are formed by photolithography, employing a mask to define the zones for the formation of patterns corresponding to the microcomponents 104 to be produced.
The term “microcomponents” as used in this document means devices or any other patterns resulting from technical steps carried out on or in the layers and that need to be positioned accurately. Thus, they may be active or passive components, simple contact points, interconnections, etc.
Furthermore, the support wafer 106 is covered by a thermal or deposited oxide layer 108 formed by oxidation of the support wafer 106, for example, in order to facilitate bonding by molecular adhesion with the first wafer 102 (FIG. 1A).
Further, a treatment is generally carried out to prepare the bonding surface 102a of the first wafer 102 and the bonding surface 106a of the second wafer 106, the treatment varying as a function of the bonding energy to be obtained (chemical-mechanical polishing, cleaning, scrubbing, hydrophobic/hydrophilic treatment, etc).
Once the wafers have been prepared, the support wafer 106 is positioned in a bonding machine 115. More precisely, the support wafer 106 is positioned on the substrate carrier 110 of the bonding machine 115 with a view to assembling it with the first wafer 102 by direct bonding. The substrate carrier 110 holds the second wafer 106 in position by means of an electrostatic system or by suction, for example.
The first wafer 102 is then placed on the second wafer 106 in order to come into intimate contact therewith (FIG. 1B). Bonding by molecular adhesion is then initiated by applying a contact force (mechanical pressure) to the first wafer 102 (FIG. 1C). Application of this contact force enables initiating propagation of a bonding wave 122 from that initiation point (FIG. 1D). The bonding wave 122 is initiated by means of an application tool 114 (for example, a TEFLON® stylus) with which the bonding machine 115 is provided.
The term “bonding wave” in this document is used for the binding or molecular adhesion front that is propagated from the initiation point and that corresponds to the diffusion of attractive forces (Van der Waals forces) from the contact point over the entire area of intimate contact between the two wafers (bonding interface).
Propagation of the bonding wave 122 over the entire bonding surfaces 102a and 106a of the wafers 102 and 106, respectively, then allows wafer bonding by molecular adhesion of the two wafers, in order to obtain a multilayer structure 112.
Once bonding has been carried out, it can be reinforced by carrying out a thermal anneal. The first wafer 102 may then be thinned in order to form a transferred layer on the support wafer 106.
However, the presence of localized bonding defects 118 at the bonding interface between the two wafers 102 and 106 has been observed, and more precisely in a region 120 located remote from the bonding initiation point 116 (FIG. 1E). Those defects correspond to zones in which the two wafers 102 and 106 have a very weak bonding force or even a complete absence of bonding.
The manufacturer does not want such bonding defects since they reduce the quality of the bond between the wafers. More generally, those defects are evidence of a non-optimized fabrication process, which reduces the attraction of the multilayer structures produced.
Thus, there is currently a need to produce multilayer structures by wafer bonding by molecular adhesion that do not exhibit such bonding defects.